LED had become widely used in lighting applications because of its high brightness, efficiency, and long life. For television and other large panel applications, hundreds of LEDs are required. Driving the LEDs efficiently is a challenge. In a large panel application, due to improvement in light scattering technology, LEDs are typically placed around the edge of a panel and still achieve acceptable light distribution. Current designs have multiple LEDs arranged in series to form a string. Multiple strings (perhaps 4-6) are used for backlighting a single television panel. Each LED string is typically driven with a high voltage (on the order of several hundred volts). For example, a string consisting of 150 in series may require 525 volts.
FIG. 1 illustrates a prior art solution for providing a high-voltage to drive LED strings. An isolated DC-DC converter is used. The term isolated refers to the fact that the input and outputs of the converter are isolated by an electrical barrier, typically a transformer. The input is V_BUS, which is normally from a rectified AC line voltage or is the output of a PFC (power factor correction) circuit. An output VCC, which is a low voltage bus, is provided to power the control circuits. Note that the low voltage bus Vc is typically very small, i.e., less than 5 volts. Vc is only intended to power the controller circuit. Another output Vs1 is provided to power the LED strings. A boost converter (comprised of S1, D1, L1, Ci1 and Co1) is applied to drive one string of LEDs. A MOSFET SD1 is in series with the LED string to achieve fast PWM dimming. A resistor Rs1 is in series with the LED string to sense the LED current during the PWM dimming on period. Multiple boost converters are applied for multiple LED strings. Thus, each block1-N has a boost converter.
There are some disadvantages with the drive scheme of FIG. 1. The boost converter must handle high voltage. As a result, the breakdown voltages of the switch S1 and diode D1 need to be higher than the LED string voltage. For example, the transistors and diodes should be able to withstand 600 volts or more. In addition, the high voltage diode D1 has a serious reverse recovery issue. This limits the LED driver efficiency. Also, the switching frequency cannot be set high, in order to reduce the size of the certain components (such as L1, Ci1 and Co1). Furthermore, the required LED power is processed with two stages from V_BUS, which lowers efficiency.